a. Field of the Invention
The present invention relates to a method of combustion, notably combustion in a heterogeneous-charge compression-ignition internal combustion engine, and to an engine for use in the method.
b. Related Art
The term “diesel engine” is used herein to refer to a compression ignition internal combustion engine in which compression initiates combustion when fuel is injected. A heterogeneous charge of fuel and air ignites in a combustion chamber because of the heat generated in the rapid compression process. This differs from Otto-cycle engines, wherein fuel and air are mixed together before being ignited by a spark plug.
A diesel engine differs from a Homogeneous Charge Compression Ignition (HCCI) engine, which uses compression ignition but with pre-mixing of fuel and air to produce a homogeneous charge. When the fuel/air mixture is compressed sufficiently it ignites spontaneously. HCCI is suitable for lean burn operation and hence can have higher efficiency than a conventional Otto-cycle engine, and lower peak temperatures which reduces NOx formation. However, HCCI is more difficult to control than combustion in conventional engines, which can cause timing problems. In contrast to a diesel engine, where ignition is controlled by the time when fuel is injected into the compressed air, or an Otto-cycle engine, where ignition is controlled by the time when a spark is generated, with HCCI there is no well-defined combustion initiator that can be directly controlled. Moreover, to achieve dynamic operation with variable work output, the control system must be able to change operating conditions such as compression ratio, inducted gas temperature and pressure, and fuel-air ratio, which can add complexity and cost. To ensure ignition and to avoid cylinder wall wetting by fuel droplet condensation the fuel employed in HCCI should have a relatively low boiling point.
U.S. Pat. No. 5,117,800 describes a method of operating a diesel or spark ignition engine which includes enriching the combustion air supply with oxygen while simultaneously adjusting the fuel injection or ignition timing of the engine to compensate for advanced combustion caused by an increased oxygen content in the combustion air. A turbocharger is used as a pump to separate air through an oxygen-producing membrane. The oxygen-enriched air is at a lower pressure, and hence is cooler, than conventionally turbocharged combustion air, so the need for an intercooler is reduced or eliminated.
U.S. Pat. No. 3,794,007 describes the use of fuel from an engine's fuel supply to heat up combustion air for a cold start. Air in a suction line or intake manifold is heated by burning fuel in a flame-suction-air-heater during startup operation of the engine under load when a poorly-ignitable fuel such as gasoline is used. The engine has a relatively low compression ratio. Burning fuel in this manner reduces fuel efficiency, and the reduction in intake air density lowers overall engine efficiency.
U.S. Pat. No. 4,333,424 discloses an isothermal engine which runs a combustion process requiring a minimum of two cylinders. The engine has a compression cylinder which compresses air for delivery via a heat exchanger to an expansion cylinder. The expansion cylinder receives the compressed air and fuel and, while combustion occurs during a power stroke, the air pressure in the expansion cylinder is reduced to atmospheric and the expansion cylinder drives a crankshaft. The process is isothermal, not adiabatic, so that the internal temperature in the expansion cylinder is kept constant or rises only a small amount during the expansion phase of the power stroke. The extra cylinders promote frictional losses.
The combustion quality of a diesel engine fuel is expressed as a Cetane Number (CN), which is defined as the percentage by volume of normal cetane (n-hexadecane) in a mixture of normal cetane and 1-methyl naphthalene which has the same ignition characteristics (ignition delay) as the test fuel when combusted in a standard engine under specified test conditions. Fuels with high CN values have low ignition delay and are suitable for use in diesel engines. Commercial diesel engine fuels typically have CN values in the range 40-55. Fuels with high CN values are typically unsuitable for Otto-cycle engines, where resistance to autoignition is desirable.
The combustion quality of Otto-cycle engine fuels is expressed as an Octane Number (ON), with high ON fuels being suitable. Typically, high CN fuels have low ON values and vice versa; hence putting diesel engine fuel in a petrol engine (or petrol in a diesel engine) can be an expensive mistake.
Fuels with low or zero cetane numbers include aromatic hydrocarbons such as toluene, and alcohols such as glycerol. Ethanol, for example, has a cetane number of about 8 and methanol has a cetane number of about 3. (M. Murphy, J. Taylor, and R. McCormick. Compendium of Experimental Cetane Numbers Data, National Renewable Energy Laboratory, 2004, NREL/SR-540-36805). The term “very low CN material” is used herein to refer to materials having a cetane number between 0 and 30. Instead of cetane number there could be used other ignition characteristics equivalent to it. Under low cetane number materials we understand combustible materials that are unable to ignite or maintain a stable operation under standard Diesel cycle mode. Very low CN materials are not regarded as suitable for use in diesel engines unless modified by admixing with a higher cetane number fuel or by the addition of cetane improvers such as polynitrate esters and amines. See, for example, U.S. Pat. No. 4,746,326 and WO 85/002194. Cetane improvers are expensive, and polynitrate esters have the drawback of being explosive.
Glycerol is formed industrially in increasing quantities as a by-product in the manufacture of biofuels for diesel engines. It would be advantageous to be able to use it, and other very low CN materials, as a fuel for a diesel engine. However, the use of heavier alcohols as diesel engine fuels is problematic. A recent review, by Mario Pagliaro, Rosaria Ciriminna, Horoshi Kimura, Michele Rossi and Cristina Della Pina From Glycerol to Value-Added Products, Angew. Chem. Int. Ed. 2007, 46, 4434-4440 states that glycerol “cannot be added directly to fuel because at high temperatures it polymerizes—and thereby clogs the engine—and it is partly oxidized to toxic acrolein”. In a recent review by Thomas Stenhede: Wartsila Green Solutions—Running Large Engines on Alternative Fuels, presented at International Seminar on Gasification, Malmo, Sweden, 10 Oct. 2008, the author summarises the state of the art: “glycerine has very poor ignition and combustion characteristics, [and] does not ignite in a diesel engine.” The high viscosity of glycerol requires heating to about 130° C. to match the viscosity for good atomisation of conventional diesel engine fuels. However, this precludes the use of cetane- or ignition-modifying additives, which typically decompose or are unstable below this temperature. For example, the predominant cetane improver 2-ethylhexyl nitrate decomposes at 120° C., which precludes combining the necessary steps of viscosity adjustment and cetane modification. Thus, attempts to date to use glycerol as a diesel engine fuel have proved unsatisfactory.